The invention relates to a controllable clutch for a belt tensioner, especially a reversible belt tensioner.
Reversible belt tensioners include a drive adapted to be coupled to the retractor axle of a belt reel by means of a clutch. During regular operation of the belt retractor the belt reel is uncoupled from said drive so that retracting or unwinding of the webbing from the belt reel is possible. In the case of tensioning the clutch is coupled to the belt reel by activation of the drive so that the webbing is wound onto the belt reel and thus is retracted.
Usually the clutch comprises a driven member having a toothing and being adapted to be coupled to a drive member via plural pawls. The pawls usually are arranged so that they are forced into the toothing by the force acting from the drive member, after the connection between the drive member and the driven element has been established. The pawls are uniformly distributed over the periphery of the driven element so that uniform force transmission takes place between the drive and the retractor axle. This is necessary to prevent additional forces due to an off-center force application. It is a drawback of those clutches that a control ensuring simultaneous engagement of all pawls is required. When the pawls are not synchronized, it is possible that individual pawls cannot be engaged and thus non-uniform force transmission to the driven element takes place.